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The Middle Mountain Metamorphic Domain of the Montana Metasedimentary Terrane, northwestern Wyoming Craton, within the northwestern Tobacco Root Mountains, mainly comprises migmatized tonalitic gneiss interlayered with amphibolitic (hornblende) gneiss, both of which are cut by metamorphosed mafic rocks. Together, these gneisses are defined as Middle Mountain Gneiss. Archean tonalitic gneiss from west of, and amphibolitic gneiss from east of, the Bismark Fault give, from chemically and air-abraded zircon grains, U-Pb ID-TIMS ages of 3325.5 ± 1.7 and 3340 Ma, respectively. These results reflect primary magmatic ages and show that the Middle Mountain Gneiss extends into the northern area of the Central Fault Block, between the Bismark and Mammoth faults. Older crustal processes in the tonalitic gneiss are evidenced by inherited grains, the oldest of which is >3460 Ma. A metabasite hosted in tonalitic gneiss in the Bismark Fault selvage zone yields a zircon age of 2468 Ma, which is interpreted as the time of metamorphism. This date and other ca. 2470 Ma dates known in the region reflect a series of thermotectonic events designated here as the Beaverhead--Tobacco Root Orogeny. Geochemical evidence in the Central Fault Block metabasites suggests that their >2470 Ma precursors evolved in a back-arc--arc-rift setting, whereas their equivalents west of the Bismark Fault were largely mid-ocean ridge basalt-related tholeiites and east of the Central Fault Block were back-arc tholeiites showing some continental affinity. The metabasite was metamorphosed, deformed, and intruded by pegmatite at 1756 Ma during the Big Sky Orogeny. This orogenic event also produced new zircon growth in Archean tonalitic gneiss. Monazite with an age of 75 Ma, found at one location, reflects nearby intrusion of the Cretaceous Tobacco Root Batholith. Le domaine meetamorphique de Middle Mountain dans le terrane metaseedimentaire du Montana, nord-ouest du craton Wyoming, nord-ouest des monts Tobacco Root, comprend surtout des gneiss tonalitiques migmatises interstratifies avec des gneiss amphibolitiques (hornblende); les deux sont recoupes par des roches mafiques metamorphisees. Ensemble, ces gneiss forment le gneiss de Middle Mountain. Un gneiss tonalitique archeeen provenant d'un site a l'ouest de la faille Bismark et un gneiss amphibolitique provenant d'un site a l'est de cette faille ont donnee des ages U-Pb ID-TIMS respectifs de 3325,5 [+ o -] 1,7 et 3340 Ma, sur des zircons pretraites par abrasion chimique ou par abrasion a l'air. Ces resultats refletent des ages magmatiques primaires et montrent que le gneiss de Middle Mountain s'etend dans le secteur nord du Central Fault Block, entre les failles Bismark et Mammoth. Des processus plus anciens de la croflte dans le gneiss tonalitique sont mis en evidence par des grains herites, dont le plus ancien date de >3460 Ma. Une metabasite encaisseee dans un gneiss tonalitique dans la zone de salbande de la faille Bismark a donne un age sur zircon de 2468 Ma, ce qui est interprete comme le moment du meetamorphisme. Cette datation et d'autres autour de 2470 Ma connues dans la region sont le reflet d'une serie d'evenements thermo-tectoniques designes ici sous l'appellation orogene Beaverhead--Tobacco Root. Les evidences geeochimiques dans les metabasites du Central Fault Block suggerent que leurs precurseurs >2470 Ma aient evolue dans un environnement d'arriere-arc--arc de distension, alors que leurs equivalents a l'ouest de la faille Bismark etait largement des tholeeiites reliesa des basaltes de dorsale meedio-oceeanique et, a l'est du Central Fault Block, etaient des tholeiites d'arriere-arc montrant une certaine affinite continentale. La metabasite etait metamorphisee, deformee et une pegmatite l'a penetreea 1756 Ma durant l'orogene Big Sky. Cet evenement orogeenique a aussi engendre une nouvelle croissance de zircons dans le gneiss tonalitique archeeen. Une monazite, agee de 75 Ma, trouveeea un site, reflete l'intrusion avoisinante du batholite Tobacco Root, au Cretace., Introduction The Tobacco Root Mountains comprise a Laramide (Late Cretaceous) block uplift in the northwestern Wyoming Province (southwestern Montana) that contains extensive exposures of Archean basement gneiss. The adjacent Great [...]

The crystalline core of the southern Black Hills, South Dakota, exposes an extensive, low-P-high-T aureole of garnet- to second-sillimanite-zone schists centered on the plutonic core of the 1715 Ma Harney Peak Granite (HPG). This paper demonstrates regional patterns of apparent ages observed for 52 [sup.40]Ar/[sup.39] Ar dates of muscovite and biotite in diverse rocks from across the ~1000 [km.sup.2] metamorphic aureole and its plutonic center. About 20 biotite dates, sampled mostly near faults, are influenced by excess [sup.40]Ar and obscure the regional trends. The remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite, with previously unrecognized, elliptical age zones centered on the main HPG pluton and its outliers. The regional pattern of [sup.40]Ar/[sup.39] Ar cooling ages indicates non-uniform slow cooling of the mid-crust between ~1600-1250 Ma. This scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite (Rb/Sr) and apatite (U/Pb), which range from 1690 to 1550 Ma and from 1700 to ~1500 Ma, respectively. To explain these results, it is likely that ambient pre-granite temperatures of the country rocks were >350[degrees]C at the ~10-14 km depth of granite emplacement, as previously proposed, and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years. Alternatively, or in addition, the HPG and inner aureole were not uplifted until ~1480-1330 Ma, whereupon they finally cooled through ~300-350[degrees]C. Keywords: Geochronology, micas, high-temperature studies, argon retentivity of micas (fieldbased), radiogenic isotopes, argon, strontium, and lead, major and minor elements, pegmatites

Abstract: The Laramide Black Hills uplift of southwest South Dakota exposes a Precambrian crystalline core of ~2560-2600 Ma basement granitoids nonconformably overlain by two Paleoproterozoic intracratonic rift successions. In the [...]

A metapelite from the easternmost Wyoming craton (Black Hills, South Dakota) has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800-1700 Ma assembly of southern Laurentia. Three deformational fabrics are recognized in oriented thin sections: an ENE-trending [S.sub.1] fabric, preserved as oblique inclusion trails in garnet porphyroblasts; a NNW-trending [S.sub.2] fabric, preserved as microlithons in the rock matrix; and a flattening fabric, [S.sub.3], which transposed [S.sub.1]-[S.sub.2] and dominates the matrix. A complex monazite porphyroblast has been analyzed in situ with the electron microprobe (Ultrachron) to constrain the timing of [S.sub.1]-[S.sub.3] fabric formation associated with monazite growth. The core of this grain uniquely preserves the [S.sub.1]-[S.sub.2] fabrics as sigmoidal inclusion trails. The mean total-Pb age of this domain is 1750 [+ or -] 10 Ma (all dates reported at 95% confidence; n = 39 spots), which is equivalent to the published [sup.207]ph/[sup.206]Pb age for the same domain. These results validate the total-Pb dating method in general and the Ultrachron in particular, for reliable age determination in low-Th monazite, and are interpreted as 1750 Ma minimum ages for the [S.sub.1]-[S.sub.2] fabrics and sequential, [D.sub.1]-[D.sub.2] collisional events that imposed them (~N-directed arc accretion and ~E-W continental collision, respectively). A higher-Th, Y rim of this same 'Rosetta' grain truncates the [S.sub.1]-[S.sub.2] sigmoid, and is associated with resorption textures in garnet porphyroblasts, coupled release of Y, and an [S.sub.3] fabric that pervasively overprinted [S.sub.1]-[S.sub.2] in the rock matrix. The mean Ultrachron date of this domain is 1692 [+ or -] 5 Ma (n = 17 spots), which is slightly younger that the published isotopic age for all monazite rims combined. These results support a ~1715-1690 Ma timeframe for localized doming ([D.sub.3]) related to granite magmatism, the onset of which has been dated independently at 1715 [+ or -] 3 Ma. The timing of post-[D.sub.3] cooling through 350 and 300 [degrees]C is constrained by [sup.40]Ar/[sup.39]Ar dates of ~1610 and ~1480 Ma obtained for separates of [D.sub.3] matrix muscovite and biotite, respectively, which are interpreted as closure ages. This study shows that fabrics in poly-deformed rocks can be dated by linking monazite spot ages to key microtextures. Further, the results of this micrometer-scale study enhance previous knowledge of local thermotectonism (Black Hills) and regional terrane assembly (Laurentia).

Polygenetic monazite grains in diverse Precambrian crystalline rocks from the Black Hills, South Dakota, have been analyzed in situ by ion and electron microprobe methods (SHRIMP and EMP), to evaluate the accuracy and precision of EMP ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections. Parallel evaluations were conducted by comparing EMP chemical and SHRIMP isotopic ages at regional-, rock-, and grain-scales. The monazite data set includes 354 EMP chemical analyses from 26 grains in six metamorphic rocks, which resolve into 54 age-composition domains, and 31 SHRIMP isotopic ages froth 13 grains in one of the rocks, with six grains microanalyzed in common by the two methods. The data set also includes monazite-bearing garnets in two of the rocks, whose isotopic compositions were analyzed using Pb stepwise-leaching (PbSL) methods. Both the EMP and SHRIMP data sets reveal a continuum of apparent monazite ages spanning a ~1790-1680 Ma timeframe, with a relatively high probability of ages at ~1755 and ~1715 Ma that correspond spatially to core and rim domains. PbSL ages of ~1742 and ~1734 Ma obtained from monazite-bearing garnet in two rocks are intermediate compared to the corresponding EMP ages, and are thereby interpreted as mixed ages. EMP data for two grains in the structurally deepest of the six rocks record ~1785 and ~1755 Ma ages in the cores and (higher-Y and lower-Th) rims, respectively, and these results are duplicated by SHRIMP ages in these and/or other grains from the same rock. Overall, the EMP, SHRIMP, and PbSL ages are internally consistent at the various scales of observation, which serves to validate EMP chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains. The EMP data set is interpreted geologically as reflecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the Black Hills. Taking the most precise EMP data at face value, it is possible to resolve the timing of the two older events at [less than or equal to] 1784 [+ or -] 4 Ma (or [less than or equal to] 1786 [+ or -] 6 Ma) and 1756 [+ or -] 3 Ma (or 1753 [+ or -] 4 Ma), with 95% confidence. These events are considered to be related to sequential episodes of N-directed thrusting and ~E-W compression associated with Paleoproterozoic crustal assembly in the mid-continent. A younger metamorphism, related to granite intrusion known to have occurred at 1715 [+ or -] 3 Ha, is dated independently at 1717 [+ or -] 2 Ma from the EMP monazite ages.

We report 207Pb/206Pb stepwise-leaching (PbSL) ages of 1762 [+ or -] 15 Ma, 1759 [+ or -] 8 Ma, and 1760 [+ or -] 7 Ma for almandine garnet, staurolite, and garnet-staurolite (combined) in an Early Proterozoic metapelite from the Black Hills collisional orogen, South Dakota. PbSL-derived 208Pb/206Pb (Th/U) trends reveal that staurolite contains inclusions of both cogenetic monazite (ca. 1760 [+ or -] 7 Ma) and older detrital zircon ([greater than or equal to]2040 Ma), whereas coexisting garnet contains only the zircon. These results and petrologic data indicate prograde sequential growth of garnet, monazite, and staurolite during tectonic burial at temperatures between [approximately]400 [degrees] C and [approximately]550 [degrees] C (i.e., well below Pb closure temperatures). We interpret the 1760 [+ or -] 7 Ma date as a maximum age for Wyoming-Superior continental collision in the Black Hills, which apparently postdated the 1800-1900 Ma Trans-Hudson orogeny in Canada, but was nearly synchronous with north-directed accretion of the Central Plains orogen in southeastern Wyoming.

The 40Ar/39Ar evidence for middle proterozoic slow cooling of the southern Black Hills, SD, was examined to determine its implications for early proterozoic P-T evolution and posttectonic magmatism. The research utilized proterozoic mica dates from metasedimentary rocks. Results revealed that uplift and crustal thinning before the Harney Peak Granite intrusion was due to differential thinning of the subcrustal lithosphere beneath the Black Hills.

The crystal-chemical analysis of micas shows that (super 40)Ar retention is less in biotite than in muscovite. The partitioning behavior of biotite and muscovite shows that the weaker interlayer bonding in biotite is due to spacious interlayer volume and fewer interlayer bonds per unit cell. Muscovite gives a more favorable electrostatic environment for Ba(super 2+) substitution than biotite. The interlayer ionic porosity affects the loss of radiogenic isotopes. The partitioning/porosity studies help geochronological predictions.

A question regarding the 1900-1600 Ma assembly of Laurentia is whether the Wyoming province (west-central United States) was part of the Hearne province (west-central Canada) prior to the Early Proterozoic Trans-Hudson orogeny, or a separate entity welded later to a Hearne-Superior continent (central Canada). New 40Ar/39Ar mica dates help to address this question by extending a Middle Proterozoic geochronologic front, long established along the southern Wyoming province, into the Black Hills of South Dakota. This suggests that previously unexplained, north-directed fold nappes ([F.sub.1]) in the Black Hills resulted from island-arc accretion to the south ca. 1780 Ma. North-northwest-trending upright [F.sub.2] folds, which formed during east-west collision of the Wyoming and Superior provinces, must therefore be younger. New 40Ar/39Ar horn-blende dates, recently published age data, and crustal heat-flow considerations further suggest that this collision began at or before ca. 1770 Ma and culminated with posttectonic magmatism beginning ca. 1715 Ma (the Harney Peak granite). This tectonic-magmatic interval is [approximately]50-60 m.y. younger than that reported for the Hearne-Superior collision (Trans-Hudson orogeny in Canada). Comparably young metamorphic dates (1810-1710 Ma) also typify the eastern and northern Wyoming province periphery (western Dakotas and southwestern Montana). Collectively, these data suggest that the Hearne and Wyoming provinces were once separate continents that were ultimately welded to the Superior province (and to each other) during distinct Early Proterozoic orogenies. Regional relationships further suggest that final docking of the eastern Wyoming province with Laurentia began during the ca. 1780-1740 Ma interval of island-arc accretion along the southern margin of the growing craton.